The present study builds on an expanding body of research on the benefits of emotionally supportive interactions, including the extent to which teachers vary in their emotional supportive interactions over time, on preschoolers' social-emotional development. Using data collected in both private and Head Start preschool classrooms, we examined associations between mean levels and variability in emotional support, teachers' stress, and children's social and emotional behaviors in the classroom. Separate analyses were conducted for Head Start and private centers as a result of descriptive analyses that indicated the settings were different. Overall, Head Start teachers showed less stress, higher levels of emotional support, and more consistency in emotional support. Furthermore, children in Head Start classrooms were less emotionally negative and aggressive. In private centers, teacher stress, variability in emotional support, and an interaction of mean level and variability in emotional support all predicted children's behavior. Private center children showed more negative emotion and aggression in classrooms with teachers who were inconsistent in their emotional supportiveness, even when those teachers were, on average, very supportive. These findings support the inclusion of variability of emotional support as an indicator of classroom quality and emphasize the importance of assisting teachers with managing classroom-related stress. (author abstract)